Surface Photovoltage-Induced Ultralow Work Function Material for Thermionic Energy Converters

Schindler, Peter; Riley, Daniel C.; Bargatin, Igor; Sahasrabuddhe, Kunal; Schwede, Jared; Sun, Steven Y.; Pianetta, P.; Shen, Zhi‐Xun; Howe, Roger T.; Melosh, Nicholas A.

doi:10.1021/acsenergylett.9b01214

Cited by 26 publications

(27 citation statements)

References 60 publications

(112 reference statements)

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“…As mentioned before, transition from FE to SCLC at low field is feasible by using low work function materials [37,38]. For φ B = 0.7 eV [37], SN model gives k = 0.04 used in Fig. 3(c), which shows a low transition E-field of about 0.3 V/nm for both D = 100 nm and D = 1 mm.…”

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confidence: 87%

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Absence of Space-Charge-Limited Current in Unconventional Field Emission

Chua¹,

Kee²,

Ang³

et al. 2021

Preprint

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For field emission (FE), it is widely expected that its emitting current density J will become space-chargelimited current (SCLC) due the built-up of charge in-transit within a gap spacing D biased at sufficiently large voltage V . In this paper, we reveal a peculiar finding in which this expected two-stage transition (from FE to SCLC) is no longer valid for FE not obeying the traditional Fowler-Nordheim (FN) law. By employing a generalized FN scaling of ln J/V k ∝ −1/V , we show the existence of a critical exponent k c ≡ 3/2 where unusual behaviours occur for k < k c : (a) Only FE at small D (no transition to SCLC even at infinitely large V ), and (b) Three-stage transition from FE first to SCLC then back to FE at large D. For any k > k c , the conventional two-stage transition from FE to SCLC will always occur for all D, which also includes the conventional FN law at k = 2. Using various unconventional FE models with k = 2, we specifically demonstrate these peculiar transitions. Under a normalized model, our findings uncover the rich interplay between the source-limited FE and bulk-limited SCLC over a wide range of operating conditions.

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confidence: 87%

“…As mentioned before, transition from FE to SCLC at low field is feasible by using low work function materials [37,38]. For φ B = 0.7 eV [37], SN model gives k = 0.04 used in Fig.…”

mentioning

confidence: 96%

Absence of Space-Charge-Limited Current in Unconventional Field Emission

Chua¹,

Kee²,

Ang³

et al. 2021

Preprint

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“…Very recently, Schindler et al 22 observed a TEC voltage boost when illuminating n-GaAs anodes by a 532 nm laser source. The authors argue that this increment in the voltage is attributed to surface photovoltage effect in an n-type semiconductor, which unpins the quasi-Fermi levels at the surface and relaxes the band-bending due to the presence of charged trap states (Figure 2a,b), thus inducing a reduction of the anode work function logarithmically depending on the radiation intensity (up to ∼0.3 eV at 10 mW/cm 2 ).…”

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confidence: 99%

“…The work function values of (4.6 ± 0.1) eV and (4.5 ± 0.1) eV have been measured for both the PV cell and the cathode surfaces, respectively, by ultraviolet photoelectron spectroscopy (UPS) under dark conditions (Figures S5 and S6). Such high work function values are far from being optimal for thermionic energy conversion and will result in very small current densities and lower output voltages than expected 22 holes are injected to the surface creating a dipole that reduces band bending and therefore shifts the Fermi level at the surface toward the vacuum level, ultimately reducing the work function (ϕ 1 < ϕ 0 ). (c) Under illumination, and according to the TIPV interpretation described in this Letter, the Fermi level remains pinned near the mid gap at the surface, and thus, the work function value is unaltered with respect to the not illuminated case (ϕ 0 ).…”

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confidence: 99%

“…(a) Fermi level pinning in n-type semiconductor creates a Schottky barrier near the surface. (b) Under illumination, and according to the conventional surface photovoltage (SPV) interpretation,22 holes are injected to the surface creating a dipole that reduces band bending and therefore shifts the Fermi level at the surface toward the vacuum level, ultimately reducing the work function (ϕ 1 < ϕ 0 ). (c) Under illumination, and according to the TIPV interpretation described in this Letter, the Fermi level remains pinned near the mid gap at the surface, and thus, the work function value is unaltered with respect to the not illuminated case (ϕ 0 ).…”

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confidence: 99%

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Photovoltaic Anodes for Enhanced Thermionic Energy Conversion

et al. 2020

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Thermionic energy converters are heat engines based on the direct emission of electrons from a hot cathode toward a colder anode. Because the thermionic emission is unavoidably accompanied by photonic emission, radiative energy transfer is a significant source of losses in these devices. In this Letter, we provide the experimental demonstration of a hybrid thermionic−photovoltaic device that is able to produce electricity not only from the electrons but also from the photons that are emitted by the cathode. Thermionic electrons are injected in the valence band of a gallium arsenide semiconducting anode, then pumped to the conduction band by the photovoltaic effect, and finally extracted from the conduction band to produce useful energy before they are reinjected in the cathode. We show that such a hybrid device produces a voltage boost of ∼1 V with respect to a reference thermionic device made of the same materials and operating under the same conditions. This proof of concept paves the way to the development of efficient thermionic and photovoltaic devices for the direct conversion of heat into electricity.

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Discovery of Stable Surfaces with Extreme Work Functions by High‐Throughput Density Functional Theory and Machine Learning

Schindler,

Antoniuk,

Cheon

et al. 2024

Adv Funct Materials

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The work function is the key surface property that determines the energy required to extract an electron from the surface of a material. This property is crucial for thermionic energy conversion, band alignment in heterostructures, and electron emission devices. This work presents a high‐throughput workflow using density functional theory (DFT) to calculate the work function and cleavage energy of 33,631 slabs (58,332 work functions) that are created from 3,716 bulk materials. The number of calculated surface properties surpasses the previously largest database by a factor of ≈27. Several surfaces with an ultra‐low (<2 eV) and ultra‐high (>7 eV) work function are identified. Specifically, the (100)‐Ba‐O surface of BaMoO3 and the (001)‐F surface of Ag2F have record‐low (1.25 eV) and record‐high (9.06 eV) steady‐state work functions. Based on this database a physics‐based approach to featurize surfaces is utilized to predict the work function. The random forest model achieves a test mean absolute error (MAE) of 0.09 eV, comparable to the accuracy of DFT. This surrogate model enables rapid predictions of the work function (≈ 105 faster than DFT) across a vast chemical space and facilitates the discovery of material surfaces with extreme work functions for energy conversion and electronic device applications.

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Surface Photovoltage-Induced Ultralow Work Function Material for Thermionic Energy Converters

Cited by 26 publications

References 60 publications

Absence of Space-Charge-Limited Current in Unconventional Field Emission

Absence of Space-Charge-Limited Current in Unconventional Field Emission

Photovoltaic Anodes for Enhanced Thermionic Energy Conversion

Discovery of Stable Surfaces with Extreme Work Functions by High‐Throughput Density Functional Theory and Machine Learning

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